Without a visual example it would be hard to help you.
I can generalize by saying that, sometimes, assets are not well behaved in that the files referenced within the MHMAT file for a given asset might not be where they belong. Thus, the MAT file will not find them and you will get the dreaded missing texture image instead.
This is rather easy to fix if you know your way around the material editor under the utilities tab in MakeHuman. The procedure is to load said asset in the geometries tab and then edit it in the material editor. Of particular interest will be the diffuse texture and ambient texture. Also, note the lightsphere if the asset is set to use one.
Once all is well, simply save the MAT file and then go into geometries and reload it.